1) Field of the Invention
The invention relates to electric lamps and, more particularly, to the mechanical and electrical connection between the conductive lead-throughs which extend from the lamp envelope and the contact terminals of the lamp cap.
2) Description of the Prior Art
Electric lamps generally have a sealed lamp envelope or bulb in which a light source, such as a filament or arc tube, is disposed and conductive lead-throughs extending from the light source through the lamp envelope to the exterior. A lamp cap or base secured on the lamp envelope and having one or more contact terminals connected to corresponding lead-throughs provides electrical connection to the light source from an associated socket.
In lamps having planar contact terminals the lead-throughs are most often fastened to the contact terminals by welding or by crimping. As used hereinafter, a "weld" is defined as a localized coalescence of metal wherein coalescence is produced by heating to suitable temperatures, with or without the application of pressure, and with or without the use of filler material. The filler material may have a melting point similar to the base metals, or lower than the base metals but above approximately 840.degree. F. This definition includes resistance welding, brazing, and braze-welding, among others.
U.S. Pat. No. 4,146,814 (Wojtowicz) discloses a single-ended miniature incandescent lamp having a two-piece metallic clip-type base. Wojtowicz teaches that the two leg portions of the clip may be connected to the lead-throughs by welding or by crimping. U.S. Pat. No. 4,687,965 (Sanders et al) shows a type 9005/9006 automotive head lamp having an insulative base of synthetic material in which planar tongue-shaped contact terminals are disposed. The lead-throughs are fastened to the terminals by welding. To increase the strength of the welds, Sanders teaches terminals having a raised ridge or rib, extending normal to the lead-through, to which the lead-through is welded. The welding current then passes through a "point-shaped" contact area between the lead-through and the rib, concentrating the welding current at one spot, and increasing the strength of the weld. This is in contrast to the prior technique of welding the lead-through to a flat surface of the terminal in which the welding current passed through a "line-shaped" larger area, which produced a weaker weld.
Weld failures have been known to occur in service, causing failure of the lamp. It is believed that the factors contributing to the weld failures include forces on the welds during lamp service, corrosion, chemical attack, and also initial defects in the welds during manufacture.
In lamps having contact terminals disposed in a lamp cap of synthetic material, such as Sanders above, the terminals are often secured within slots in the cap by means of barbed hooks and/or resilient tongues. It has been found that in service, forces on the lamp cap and the associated socket cause movement of the terminals with respect to the lamp cap and lead-throughs. This movement has been known to cause failure of the welds between the terminals and lead-throughs, even with the improved weld taught by Sanders.
The welds may also be weakened by corrosion from water, dirt, dust, and/or road salts which may infiltrate the lamp cap during service. In lamps where the welds are protected from the environment, as in the Sanders lamp above in which the lead-throughs and terminals are protected by a cover and a synthetic sealing material, such as silicone rubber, it has been found that chemicals, such as ascetic acid, from the sealing materials chemically attack the welds, contributing to their failure.
Additionally, the strength of the welds may be limited initially because the available materials for the lead-throughs and the contact terminals may be restricted. For example, for 9005/9006 type automotive headlamps as shown in Sanders, SAE standard J580 limits the permissible voltage drop in the contact terminals, with the result that the lead-through and contact terminal materials may be less than optimally compatible from a welding standpoint.
Another factor which contributes to unsatisfactory welds when resistance welding is used is equipment and/or operator error in aligning the welding electrodes with the lead-through, causing insufficient welding current to flow through the contact area between the lead-through and the contact terminal.
Accordingly, it is an object of the invention to provide an electric lamp having a strengthened mechanical and improved electrical connection between the conductive lead-throughs and the contact terminals of the lamp base or cap.
It is another object of the invention to provide an electric lamp in which the reliability of the connection between the lead-throughs and the terminals is increased by reducing the possibility of operator/ welding equipment error.
It is yet another object of the invention to provide an improved connection which is cost effective.